The present invention may be applicable to any type of centrifuge or separator including a conveyor-type apparatus therein. Probably the most common type of centrifuge including a conveyor is a decanter centrifuge. The description contained herein relates to the specific structures of a typical solid bowl decanter centrifuge. This description, however, is not limiting to the scope of the present invention as presently contemplated.
A decanter-type centrifuge comprises a rotating bowl, typically having a cylindrical portion and a frusto-conical end portion. The rotation of the bowl creates a centrifugal force which separates a liquid feed mixture into its constituent parts. The feed mixture within the bowl forms a cylindrical pond, with a ring or layer of separated heavy material adjacent the inside of the bowl wall and a ring or layer of lighter material radially inward of the heavy material layer.
The terms "heavy phase" and "light phase" are often employed to describe materials which are separable from the feed mixture by the application of centrifugal force. In a decanter centrifuge having a conveyor, the light phase material will usually be a liquid and the heavy phase material will usually be a mixture of solids which may also include some liquid. The liquid feed mixture or slurry introduced into the bowl generally has a specific concentration of suspended solids or other insoluble material therein. These solids are generally concentrated by the centrifugal force to form a heavy phase or mixture within the rotating bowl, including coarse solids, fine solids and liquid. Because of the varying degrees in density of the solids as well as the varying degrees of centrifugal force acting on those solids within the bowl, the concentration of the separated heavy phase may vary within the bowl. The concentration of the heavy materials that do not settle from the liquid material also varies.
In a decanter centrifuge, a screw conveyor rotates inside the bowl at a slightly different speed from the bowl. The flights of the screw conveyor push the separated heavy phase along the inside of the bowl wall towards the conical end of the bowl. Discharge ports for the separated heavy phase are located at the small diameter of the conical bowl portion. The separated light phase liquid is discharged by flowing from the cylindrical pond through separate discharge ports. The light phase liquid discharge ports are located, typically, at the opposite end of the bowl from the heavy phase discharge ports.
Separation of the heavy phase materials from the feed mixture is a function of the residence time of the mixture in the bowl, a function of the feed rate, difference in specific gravity of the solids of the heavy phase and the liquid of the light phase, and the ability of the centrifuge to separately discharge the heavy and light phase materials. The purpose of the decanter centrifuge is to separately discharge a concentrated heavy phase and a clarified liquid. In order for the heavy phase to be discharged, it must be moved up the incline of the conical end portion of the bowl, called the beach, against the centrifugal force component acting in the opposite direction downward along the beach (away from the heavy phase discharge).
Certain structures have been defined for introducing a rinse liquid within a centrifuge to wash contaminates from the surface of concentrated heavy phase/solids. For example, Shapiro U.S. Pat. No. 4,654,022 defines a chamber formed on the trailing surfaces of the conveyor flight for receipt of a rinse liquid. The rinse liquid is directed from the chamber through a plurality of orifices within the conveyor flight. In addition, an overflow passageway is provided adjacent to the top of the conveyor flight adjacent the conveyor hub. The overflow passageway cooperates with a baffle, positioned forward of the flight, to direct liquid along the front surface of the flight.
Kowata U.S. Pat. No. 3,302,873 shows a screw conveyor including a series of flow passageways extending radially outwardly through the conveyor flight from the conveyor hub. The rinse liquid is directed into the bowl of the centrifuge from orifices positioned at the distal end of the conveyor.
Redeker, et al. U.S. Pat. No. 4,496,340 shows a screw conveyor within a centrifuge having a liquid distribution channel on the radially-inward surface of the conveyor flights adjacent to the conveyor hub. The rinse liquid is fed from the conveyor hub into the channel and is directed onto the front surface of the conveyor by means of an overflow channel or a series of directional nozzles.
In certain processes for which a centrifuge is applied, it is sometimes necessary to rinse the heavy phase cake in the beach area of the centrifuge. Within these processes, it may be desirable to direct the rinse liquid to a radius within, or outside of, the normal level of the heavy phase cake rather than to direct it onto the top surface of the cake (such as in the Redeker patent or by the overflow channel in the '022 patent discussed above). The present invention is directed at least in part to accomplishing this result.